Environmental awareness, legislative measures, and public demand for environmental sustainability are leading to an increased interest in plastics recycling. Plastics material recycling is important for a wide number of reasons, including diminished use of petroleum resources, reduction of environmental pollution, preservation of landfill space, conservation of energy, and enhanced post-consumer reuse.
Poly(ethylene terephthalate) (PET), a thermoplastic polyester that is used in clothing fibers and fabrics, carpeting, packaging films, food containers, and beverage bottles is one of the most common post-consumer materials to exist in landfills. There are two conventional methods for processing post-consumer PET: mechanical recycling and chemical recycling.
Mechanical recycling, the most commonly practiced recycling method, produces a somewhat impure recycled material; for this reason, it is often used to form products other than beverage bottles. Mechanical recycling entails melt-processing and remolding post-consumer PET. Without rigorous sorting and cleaning of the incoming PET product, mechanical recycling produces a recycled PET product that lacks many of the desirable mechanical and optical properties of the original PET product. For example, the mechanical recycling melt process deteriorates the intrinsic viscosity of the PET. Further, any metals, dyes, and color contaminants present in the PET may carry over into the recycled product. By contrast, chemical recycling of PET can yield high quality PET via the chemical degradation of PET followed by repolymerization of the formed monomers. The quality of PET produced from chemical recycling of mixed post-consumer PET makes this recycling method suitable for the conversion of consumer waste PET products into high value post-consumer PET products.
Chemical recycling of PET involves the depolymerization of post-consumer PET into product monomers and oligomers. Catalysts used to carry out the depolymerization reactions include NaOH, KOH, Zeolite, metals and strong organic bases (such as guanidines and amidines). Because metal catalysts are non-biodegradable, they are considered pollutants and additional processing steps are required to remove the metals from the resultant monomers and/or oligomers. Further, if metal were to remain in the recycled monomers and/or oligomers, their presence would cause potential problems during the repolymerization of the monomers and/or oligomers back to PET. While strong organic bases are effective at catalyzing polyester depolymerization, they are relatively costly, not easy to recycle, can cause color formation in the product monomer, and are subject to air oxidation and neutralization by salt formation; characteristics that render strongly basic organocatalysts poor candidates for polyester depolymerization reactions. Another drawback to the use of metal-containing catalysts, or strongly basic organocatalysts, is that they saturate the ion exchange resins that may be used to remove residual traces of the PET polymerization catalysts in post-reaction purification, thus rendering the resins unusable for further purification procedures and increasing the frequency of regeneration cycles.
To date, there are no depolymerization catalysts that allow for effective and economical recycling of PET. Current initiatives in the chemical recycling of PET are focused on the following factors: environmentally safety, economic feasibility, ready recyclability, and industrial applicability.